Rational Regulation of the Linkage Polarity in Imine-Linked Covalent Organic Frameworks for Efficient Photocatalytic Nitrogen Reduction to Ammonia

Imine-linked covalent organic frameworks (COFs) have gained considerable attention for photocatalytic reduction of nitrogen (N2) to ammonia (NH3). However, the intrinsic polarization of the linking C═N bonds leads to a high energy barrier for π-electron delocalization, giving rise to high exciton binding energy and slow charge separation/transfer efficiency. In this study, the C═N linkages of the imine-linked COFs TA-BT were intentionally protonated, oxidized, and methylated by a facile postmodification strategy, thereby yielding three novel polarized COFs with different linkage polarity, namely, TA-BT-H, TA-BT-O, and TA-BT-I. The resulting COFs exhibit a higher dielectric constant and lower exciton binding energy as compared to pristine TA-BT, accelerating charge separation/transfer. Without adding sacrificial agents or cocatalysts, the NH3 yield rates of TA-BT-H, TA-BT-O, and TA-BT-I reach 376.5, 498.2, and 320.8 μmol g–1 h–1, significantly higher than pristine TA-BT (185.6 μmol g–1 h–1) and surpassing most previously reported photocatalysts. The mechanism study indicates that the polarized COFs have undergone an alteration in the N2 adsorption sites, thereby enhancing adsorption and activation. Furthermore, the N2 reduction pathway favors an alternating hydrogenation mechanism. This finding highlights the significant potential of the linkage polarity in imine-linked COFs for photocatalytic N2 reduction to NH3.